1. Field of the Invention
The present invention relates to adhesive compositions which provide a high bond strength to metals and particularly to noble metals, a method of bonding metals using such an adhesive, and articles in which a metal is bonded using such an adhesive. More particularly, the present invention relates to adhesive compositions which provide a high bond strength to various metals used in the dental field.
2. Description of the Background
Adhesives based on monomers such as acrylic compounds and epoxy compounds can be allowed to cure at room temperature and atmospheric pressure in short periods of time, offering good workability, and as such have been in prevalent use for the bonding of metals. However, depending on the intended use, these adhesives may exhibit serious drawbacks. Thus, if the bonded product is kept exposed to water, the bond strength suffers a sharp decline, so that these adhesives can hardly be used in applications demanding water resistance.
Recently, in the field of dental materials, various attempts have been made to provide adhesives for metals with improved bond strength water resistance, and these efforts have been rewarded by some degrees of success. For example, a dental adhesive composition containing a phosphoric acid ester compound as disclosed in Japanese laid-open Patent Application Kokai No. 21607/1983 exhibits an excellent water-resistant bond strength to base metals, such as iron, nickel, chromium, cobalt, tin, aluminum, copper, titanium, etc., and base metal alloys containing such elements as major ingredients, and has actually been in use clinically as a dental cement. However, the adhesive composition according to this reference is inferior in the water resistance of the bond strength to noble metal alloys containing gold, platinum, palladium or silver, e.g. inlays, crowns and bridges, as compared to that to base metal alloys.
It has been therefore found necessary, for imparting an adequate water resistance of bond strength, that such noble metal alloy products should be surface-treated by tin plating or oxidation.
It was recently reported in the Japanese Society for Dental Materials and Devices, 5, 92-105 (1986) that coating of N-(4-mercaptophenyl)methacrylamide as a primer to a noble metal markedly improved the water resistance of adhesion of MMA-TBB resin to the noble metal.
Further, Japanese laid-open Patent Application Kokai No. 110523/1976 describes that a varnish based on a methacrylic acid ester is improved in its bond strength to a glass or an aluminum plate by addition of 3-(isopropenylthiocarbonylthio)propyltrimethoxysilane to the varnish. However, when the adhesive containing 3-(isopropenylthiocarbonylthio)propyltrimethoxysilane is applied to noble metals or noble metal alloys, the resulting bond strength is not sufficient for practical use.
As mentioned above, when a noble metal or its alloy is bonded to the tooth or a dental material with the adhesive disclosed in Japanese laid-open Patent Application Kokai No. 21607/1983, the metal surface must be previously treated by tin plating or oxidation and such a procedure requires skill and is time-consuming. This trouble can be obviated by using, instead, the above-mentioned N-(4-mercaptophenyl)methacrylamide-containing adhesive, but there still remains the problem that the water resistance of the adhesion is not adequate for practical use.
Thus, there remains a need for a primer for the surface conditioning of metals, particularly noble metals, prior to adhesion of a metal to another metal or other material which results in a high bond strength with a high water resistance, and there also remains a need for an adhesive for metal-to-metal bonding or metal-to-other material bonding which provides a high bond strength with a high water resistance.